The BasicPlatform Plugin

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Summary

The BasicPlatform plugin was created to provide backward compatibility in
DIRSIG5 for the sensor collection capabilities in DIRSIG4.

Input

The inputs to this plugin are the DIRSIG4 era platform, motion (either a
GenericMotion or a FlexMotion
description) and a tasks file. This allows the plugin to emulate the
sensor collection capabilities in DIRSIG4.

Implementation Details

Spatial and Temporal Sampling

This sensor plugin performs spatial and temporal sampling differently than
DIRSIG4 did. Specifically, DIRSIG4 used discrete sampling approaches
to separately sample the spatial and temporal dimensions of flux onto a
given pixel and DIRSIG5 simultaneously samples the spatial and temporal
dimensions with uniformly distributed samples.

In DIRSIG4, the amount of sub-pixel sampling was controlled on a per focal
plane basis using rigid N x M sub-pixel sampling. Furthermore, delta
sampling vs. adaptive sampling was a choice for the user. In contrast the
BasicPlatform plugin uniformly samples the detector area and the number of
samples used for each pixel is adaptive and driven by the convergence
parameters. In general, the entire spatial response description in the
input file is ignored by this plugin.

The DIRSIG4 temporal integration feature included an integration time and
an integer number of samples to use across that time window. The temporal
sampling approach brute-force resampled the entire detector for each sample,
which resulted in a proportional increase in run-time. The DIRSIG5
BasicPlatform plugin ignores the number of temporal samples and instead
uniformly distributes the ray times across the integration time window.

Experimental Features

This plugin includes a few experimental features that may become permenant
features at some point in the future. Please utilize these features with
caution since the feature maybe removed or replaced by a more mature
version.

Point Spread Function (PSF)

The user can currently describe the modulation transfer function (MTF) of
the optical system via a single Point Spread Function (PSF). Note that
a single PSF is provided and hence, this is currently not a position
dependent PSF. This feature is enabled by manually editing a DIRSIG4
.platform file and injecting the <psf> XML element in the <focalplane>
element (see example below):

As an unofficial/experimental feature, the <psf> element will
get deleted if the user loads and saves the .platform file in
the graphical user interface.

The PSF is supplied as a grayscale image file (JPG, PNG, TIFF, etc.
via the <image> element) and the user must define the extent of the
image in pixels widths (via the <scale> element). In the example above,
the Airy disk pattern in the PNG file has an equivalent width of 10
pixels.

The PSF image is used in a 2-step importance sampling scheme to emulate the
convolution of the PSF with the pixel area. In general, more samples per
pixel are required to faithfully reproduce the impacts of the PSF on the
output.

Active Area Mask

The user can supply an "active area mask" for all the pixels in the
array via a grayscale image file. The brightness of the pixels are
assumed to convey the relative sensitivty across the pixel area.
A mask pixel value of 0 translates to zero sensitivity and a mask
pixel value of 255 translated to a unity sensitivity. Hence, areas
of the pixel can be masked off to model front-side readout electronics
or to change the shape of the pixel (e.g. circular, diamond, etc.).
The mask is used to overrride the uniform pixel sample with an importance
based approach. Therefore, the active area image can contain gray values
(values between 0 and 255) that indicate the pixel has reduced sensitivity
in that specific area.

The active area mask is supplied via the <activearea> XML element within
the <focalplane> element (see example below):

As an unofficial/experimental feature, the <activearea> element
will get deleted if the user loads and saves the .platform file
in the graphical user interface.

The <image> element specifies the name of the 8-bit image file
(JPG, PNG, TIFF, etc.) used to drive the pixel sampling.

Note that this pixel area sampling can be combined with the PSF feature
described previously.

Usage

The BasicPlatform is implicitly used when the user launches DIRSIG5
with a DIRSIG4 era XML simulation file (.sim). To explicitly use
the BasicPlatform plugin in DIRSIG5, the user must use the newer JSON
formatted simulation input file (referred to a JSIM
file with a .jsim file extension). At this time, these files are
hand-crafted (no graphical editor is available). An example is shown
below: